Shawnee Smith was pregnant during filming, but kept it a secret from everyone, including the director and producers. Her daughter gave the secret away one day during lunch, but only to the director/writer Darren Lynn Bousman.
This film originally came from a script by the director Darren Lynn Bousman which was called "The Desperate". After trying for years to get it made but being told repeatedly that it was too violent, finally a company wanted to do it because they suspected Saw (2004) which was becoming a hit at Sundance might blow out big and they wanted to capitalize on its success. Some producers even described Bousman's script as "too Saw-ish". Just before he was about to close a deal to make the movie, Saw (2004) opened huge at the box-office and the next day he received a call, and the producers asked if he could change it around to "Saw II". Leigh Whannell (who wrote the first "Saw") was then brought on a little later to help Bousman with creating his original idea into a proper sequel to Saw (2004).
The advertising posters for the film had to be recalled on orders from the MPAA because the two severed fingers forming the "II" in the posters did not meet guidelines for film advertising. The new poster makes it more obscure. Also, posters had listed the film as being rated R when at the time the film still hadn't gone before the MPAA for a rating.
The key hidden inside the needle pit would open the steel door in the room. Although most of the audience believe that it would lead to the exit, the director confirmed in the commentary that it would reveal two more syringes with antidote.
Jigsaw's puppet was originally controlled by fishing line (in Saw (2004)). This time around, the crew had a slightly larger budget and decided to completely redo the puppet and make it mechanically controlled.
(at around 33 mins) According to the DVD commentary, Xavier's retort to Addison was improvised each take, and got progressively more vulgar. The crew picked the line "The only door you know how to open...", thinking it was the most clever and funny.
(at around 41 mins) When Obi popped out of the furnace he hit Glenn Plummer's (Jonas's) eye. They had to stop filming for half an hour to make sure he was okay. He can be seen turned around in one shot, and holding his eye in another.
When shooting the Needle Pit scene a handful of real needles fell into the pit thus causing the crew to halt filming and find the needles before filming could commence. Therefore they were literally for needles in a needle pit.
There's a reference to the Wes Craven directed horror film, "The Last House on the Left" when Jigsaw is taking Eric to the house. Eric ask which way to go, and Jigsaw replies with "It's the last house on the left." Monica Potter would also star in a remake of "The Last House on the Left" in 2009.
The winner of Lionsgate's Saw II Haunted House Contest went to The Mansion of Terror in Austin, TX. This haunted attraction was featured in a documentary by Jav Rivera entitled The Scare: The Mansion of Terror Documentary (2007) which went on to win several awards. Additionally, Javier was the photographer for Mansion of Terror's contest entries.
The trivia items below may give away important plot points.
at around 1h 8 mins) The "hand trap" containing the syringe actually has a padlock with a key in it; you can see it out of focus at the top of the screen when Addison puts her hands into the trap. The filmmakers deliberately included this as a way of showing that Jigsaw knew the characters would rush in without checking the trap for a simple solution.
at around 1h 19 mins) At the end of the film, Xavier cuts his number off the back of his neck and puts it in his pocket. On the DVD commentary, it is revealed that the script called for Xavier to merely toss the skin away, but when Franky G chose to put it in his pocket, the crew loved it so much they used it instead.
at around 1h 13 mins) When Jigsaw is giving directions to the fake house, he tells Detective Matthews that it's the last house on the left. This is a reference to the movie The Last House on the Left (1972), in which the parents of a murdered teenager torture and kill the murderers.
at around 1h 17 mins) The final used syringe found on the floor when Eric discovers the contraption door leading to the basement, was allegedly the antidote that saved Daniel's life. Amanda made sure that Daniel would not be poisoned by even the slightest dose of gas he could have inhaled inside the house.
There was a lot of backstory about the contestants of the game which was deleted mostly for pacing reasons. Addison was indeed a prostitute which was merely implied in the finished film (she can also be seen in a flashback in Saw IV (2007) where she tries to seduce a potential customer). She'd had sex with Detective Matthews in the back seat of a police car, but they were discovered and the scandal led to Eric's demotion. However, he made sure that Addison would be incarcerated. The first victim was called Gus and he was an embezzler of money. The needle in the Glass Box trap was intended for him with the symbolic reference of the proverb "trying to reach the cookie jar". Laura was imprisoned because she was a shoplifter. Obi was meant to reach a second portal at the back of the furnace crawling through the flames but he would be badly burned during the process accounting for the tape which stated that he would have to confront the devil.
Before deciding to place Addison in the razor box trap, the creators and writers discussed (on a DVD commentary) that they had originally intended for her to end up in a different trap. Addison would wake up shackled at the wrists to a chair. Razors would be planted against her wrists to dig into her flesh if she tried to remove her hands. Her tape would talk about her using her looks to survive (implying she is a prostitute), stating that she would have to sacrifice and scar them if she wanted to escape. The only way to release her hands from the razor shackles would be to burn her face against a waffle iron-like grill that was in front of her. After struggling and cutting up her wrists, Addison would then finally slam her face against the grill, releasing her cuffs and letting her go free, but with a horrendously scarred face. The trap was not used, but a variation was used in Saw IV (2007), with the victim having to push their face through two columns of knives. If done successfully, this would release the wrist locks, but leave the victim with a severely lacerated face.
Various items from the house can be seen in Jigsaw's lair when Detective Matthews is questioning him, hinting that the video feed is old. According to the director, the rope and the paddle that Amanda picked up were both in the lair, but very hard to spot. The obvious item is the picture with the "X marks the spot" on it, prominently seen behind Jigsaw in many scenes.
The creators and writers had originally thought of having Amanda be forced to dig through a bathtub full of the syringes, but decided it wouldn't be enough, instead wanting it on a grander scale. After thinking of the pit in the middle of the room, they intended for Amanda to land up to her neck in the needles as if it were a pool, but realized that it would be impossible for her to sink into such a pool, along with the fact that getting enough needles would have taken too long as it took the production team a long time to get as many needles as they had.
Instead of cutting his eye open to get the key, Michael was originally going to be supplied with the key in one of his hands, but would be strapped to the chair at the wrists. Above the hand with the key was a razor, which would dig further into his flesh the more he reached the key up to the locks on his harness.
Despite popular belief, Daniel was poisoned, while it implies Amanda was not. While Amanda is fine throughout the entire film, Daniel is seen coughing up blood in the basement bathroom. While Daniel's father is checking the first room, you can see the safe open, suggesting Amanda used the antidote inside to save Daniel.
A subplot missing from the final cut; Eric Matthews had an affair with Detective Kerry. This precipitated the end of his marriage and led to the strained relationship between him and Daniel. Also, Eric's wife won the custody of Daniel quite easily. The only remaining hint of this backstory is when Kerry remarks that she is not going to let Eric lose his son for the second time.
at around 12 mins) Instead of the step shooting forward and breaking the SWAT officer's shins, a blade had originally been intended to shoot forward and slice off the first SWAT officer's legs once said rigged step had been activated.
While the creators and writers were thinking of Gus' trap, Darren Lynn Bousman suggested a trap in which someone would have to reach into a jar (as Gus embezzled money, and his tape would say something along the lines of him always putting his hand in the proverbial cookie jar). If they weren't able to retrieve the antidote, they would be forced to lose their hand. This finally developed into the razor box trap, which was then used on Addison instead. When Addison walks into the room, you can see her take an envelope with the tape inside, meant for Gus. Tearing it open, she finds the tape, but tosses it aside, as she doesn't have a tape recorder to play it.
It is assumed there would be at least seven antidotes in the house, one for each person, excluding Amanda. Only six however are mentioned/shown. One in the safe, which is presumably used on Daniel in the end. Then there are two in the furnace with Obi, two in the needle room, and one in the razor box trap.
Before settling on the final version of the furnace, the creators and writers of originally thought of having Obi's tape recorder attached to the collar of a pit bull in the basement. They decided against it, as it would lead to cruelty to animals, and instead attached it to a corpse-like mannequin. According to storyboards for this scene, Obi would have only had to reach for one antidote, while a key took the place of the second antidote.
Jigsaw states that if Detective Matthews plays along with his game and listens long enough, he will find his son Daniel "in a safe and secure state". He is already providing a clue to the boy's location, as Daniel is literally found in a safe at the end.
Despite the implication that there are seven antidotes in the house (one for each of the participants) only six are ever seen or identified: one in the safe, two in the needle pit, two in the incinerator, and one in the razor box.